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The Discovery of Radium

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The Discovery of Radium The discovery of radium How are new elements discovered? These days, it is through deliberate, concerted effort in a known direction: the only empty spots in the periodic table are all at the high end, at atomic numbers above 105 (or 108, or whatever the current value is). Physicists know that if they bombard heavy elements with energetic nuclei, they may very occasionally create a very heavy nucleus; and, if such nuclei should last long enough to build up a macroscopic sample of material, they may measure the properties of a new element. It is hard work, but follows a clear path with a well defined goal. Back in the old days, however, life was not so simple. The periodic table was still a work in progress: there were plenty of gaps and holes, as well as puzzling inconsistencies. More important, there was no theory to guide scientists in their work: the nature of atoms and nuclei was not yet understood at the turn of the twentieth century. Chemists and physicists did not know that there were exactly 30 distinct elements between hydrogen and zinc, or that helium would have exactly five siblings among the (stable) noble gases. Let us look at the discovery of the elements polonium and radium: they were tentatively announced by the team of Pierre Curie, Marie Curie, and Gustave Bemont in 1898. It was four more years, however, until Marie Curie was able to isolate a pure enough sample of radium to measure some of its properties. Background: the era of invisible rays In 1895, Wilhem Conrad Roentgen, a physicist working at the Unversity of Wurzburg in Germany, was investigating the properties of a "Crookes tube": an evacuated glass tube through which a strong electric current is passed at high voltage. He noticed some curious phenomena which were somehow connected to the operation of the Crookes tube, but took place far from it. Thanks to Dream Anatomy for a copy of Roentgen's X-ray of his wife's hand. Read more about Roentgen's discoveries Roentgen realized that invisible rays were coming from the Crookes tube, flying through the air, and interacting in novel ways with a wide variety of materials. He named the rays X-rays. People around the world were excited by the discovery of these rays, and the fascinating properties they had. Was it possible that there could also be other "rays" in the world? Radioactivity Just a year later, French physicist Henri Bequerel noticed a peculiar little event: he left a small piece of uranium (wrapped up inside a piece of paper) sitting on a photographic plate in a drawer for several weeks. When, on a lark, he developed the plate, he found a dark spot at the location which had been directly under the uranium. Somehow, the uranium had exposed the photographic plate, even though it had been covered in opaque material. Could this be another sort of invisible ray? In 1897, a graduate student at the University of Paris named Marie Curie decided to investigate one aspect of this new phenomenon for her doctoral dissertation. She was twenty-nine years old and had just given birth to her first child, Irene, during September of that year. "What," she asked, "was the nature and origin of this radiation?" She would continue this investigation for the rest of her life. Her husband, Pierre Curie, a physicist at the University, was her partner in the work until his death in 1906. The first step was to look at the source of the radiation. It was known that materials containing compounds of uranium and thorium were radioactive (a term coined by Marie Curie). The Curies decided to look harder ... The source(s) of radioactivity How can one measure radioactivity in a quantitative manner? One instrument used by the Curies was a simple device: Two metal plates are separated by a gap of a few centimeters of air. Marie Curie described hers as follows: (Diameter of the plates 8 cm; distance 3 cm.) A difference of potential of 100 volts was established between the plates. The current which passed through the condenser was measured in absolute value by means of an electrometer and a piezo-electric quartz. From "Rayons Emis par les Composes de L'Uranium et du Thorium," Comptes rendus, Vol 126, April 1898 When a voltage is applied across the plates, no current runs. Why not? Because air isn't a conductor, of course. However, if one can somehow change the air so that it WILL conduct electricity, then the size of the current will indicate the conductivity (or resistivity) of the air. Bombarding the air with energetic particles can ionize some of the molecules, turning them into charged entities which will carry a current. The Curies would simply cover one of the two plates with some material. If no current ran through the system, it was inert ... ... but if a current did run, then the material was radioactive. The size of the current provided a quantitative measure of the radio-activity of the substance. This setup produced currents which were very small: a sample of pure uranium gave a current of 24 picoamps (that's 24 x 10-12 amperes), for example. Marie Curie reported a very puzzling result: pure uranium and thorium did not produce as much current as certain ores (pitchblende and chacolite) from which they were refined. Material picoamps Uranium 24 Black oxide of uranium 27 Hydrated uranic acid 6 Pitchblende from Johanngeorgenstadt 83 Pitchblende from Joachimstahl and from Pzibran 67 Natural chacolite 52 Artificial chacolite (see below) A partial copy of the table in Rayons Emis par les Composes de L'Uranium et du Thorium Chacolite is a mixture of copper and uranium phosphates. When Marie Curie made her own "artificial chacolite" by combining pure samples of copper and uranium with other materials, she found the resulting compound to have no more radioactivity than other uranium compounds. So, why were some of the raw materials stronger sources of radiation than pure uranium and thorium? The Curies speculated that there might be an extra, very strongly radioactive component in the raw mixtures which was eliminated by chemical processing. They set out to find this postulated, very radioactive substance which was hiding in the raw ores. Naming Polonium and Radium During 1898, the Curies continued their chemical experiments, trying always to identify the portion of the uranium ore which was most strongly radioactive. In December, 1898, they announced that there were apparently two highly radioactive substances hidden in the ore. They explain: Two of us have shown that by purely chemical processes one can extract from pitchblende a strongly radioactive substance. This substance is closely related to bismuth in its analytical properties. We have stated the opinion that pitchblende may possibly contain a new element for which we have proposed the name polonium. The investigations which we are now following are in accord with the first results obtained, but in the course of these researches we have found a second substance strongly radioactive and entirely different in its chemical properties from the first. In fact, polonium is precipitated out of acid solutions by hydrogen sulphide, its salts are soluble in acids, and water precipitates them from these solutions; polonium is completely precipitated by ammonia. The new radioactive substance that we have just found has all the chemical aspects of nearly pure barium: It is precipitated neither by hydrogen sulphide, nor ammonium sulphide, nor by ammonia; the sulphate is insoluble in acids and water; the carbonate is insoluble in water; the chloride, very soluble in water, is insoluble in concentrated hydrochloric acid and in alcohol. Finally this substance shows the easily recognized spectrum of barium We believe nevertheless that this substance, although constituted for the greater part by barium, contains in addition a new element which gives it its radioactivity and which moreover is very close to barium in its chemical properties ... From Sur Une Nouvelle Substance Fortement Radio-Active, Contenue Dans La Pitchblende by M. P. Curie, Mme. P. Curie and M. G. Bemont, Comptes rendus, Vol 127, Dec 1898. The barium-like substance was by far the strongest source of radioactivity in the ores: they measured an activity level nine hundred times higher than that of uranium! Let's stop for a moment to look at the actual levels of radio-activity for samples of uranium and radium. Over periods short compared to the half-life of two substances, the relative number of decays is equal to the inverse ratio of their half-lives. Suppose we have two samples of material, uranium and radium, with an equal number of atoms in each. Q: What is the half-life of the most common isotope of uranium, U-238? Q: What is the half-life of radium-226? (This is produced in the sequence of decays starting from U-238) Q: What is the ratio of (number of decays from the sample of radium during a day) to (number of decays from the sample of uranium during a day)? Q: Was the Curie's sample of radium pure at this point in 1898? A spectrum of this very active substance, taken by Eugene Demarcay, showed a number of strong lines due to barium and lead; but there was also a strong line at a wavelength of 3814.8 Angstroms. Demarcay wrote: It does not seem possible to me that this line can be attributed to any known element ... Neither barium nor lead from elsewhere [i.e. from sources other than the Curies' material], as I have assured myself, give any line which coincides with it.
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